1. Field of the Invention
The present invention relates to a method of depositing a textured layer of transparent conductive material on a substrate. More particularly, the present invention relates to a method of controlling the texture of a tin oxide layer independently of the overall thickness of the layer.
2. Description of the Related Art
Many photovoltaic devices require the fabrication of a layer of transparent conductive material on a transparent substrate to provide an electrically conductive element on the light-entry surface of a semiconductor body that does not substantially interfere with propagation of light into the semiconductor body through the light-entry surface.
FIG. 1 shows a photovoltaic device 10 comprised of a plurality of series-connected photovoltaic cells 12 formed on a transparent vitreous substrate 14 and subjected to solar radiation 16 passing through substrate 14. Each photovoltaic cell 12 includes a front electrode 18 of transparent conductive oxide, a photovoltaic element 20 made of a semiconductor material, such as for example, hydrogenated amorphous silicon in a PIN structure, and a back electrode 22 of a metal such as aluminum. The back electrode 22 of each cell 12 is connected to the front electrode of an adjacent cell 12 by an interconnect portion 24.
Front electrodes 18 typically are formed by depositing a film of transparent conductive oxide, such as tin oxide (SnO.sub.2), on a glass substrate 14 by chemical vapor deposition. The tin oxide film then is divided into a plurality of front electrodes 18 by removing narrow strips of material between the desired front electrodes. One such method for removing the strips of tin oxide is laser ablation.
The tin oxide film fabricated on the supporting substrate must, of course, permit light to propagate through to the photovoltaic semiconductor material (photovoltaic elements 20 in FIG. 1). The tin oxide layer also serves to conduct the charge carriers generated in the photovoltaic material out of the device. It is further desirable to provide a textured surface on the deposited tin oxide film to scatter light passing into the photovoltaic elements. Scattering the light at the interface between the tin oxide front electrodes 18 and the photovoltaic elements 20 increases the effective path length of light in the photovoltaic elements, thus increasing the probability that a photon will be absorbed and produce a free charge carrier
As applied to the surface of a transparent conductive oxide layer, a "textured" surface has a dominant local peak-to-valley variation in height greater than about 0.2 micrometers. The textured surface is comprised of pyramidal microcrystallites ranging in size typically from about 0.1 to 1.5 micrometers in diameter as measured on scanning electron microscope (SEM) photographs.
One conventional method for fabricating a textured layer of tin oxide on a substrate is disclosed in U.S. Pat. No. 4,532,537, issued to Kane. Kane discloses a method of depositing a layer of textured tin oxide on a glass substrate by chemical vapor deposition from an atmosphere including tin, oxygen, hydrogen, and a conductivity modifying dopant such as fluorine. The primary reactants are tin chloride (SnCl.sub.4) and water (H.sub.2 O), both of which are introduced by bubbling nitrogen (N.sub.2) through the reactants. The substrate is held at an elevated temperature lower than the temperature at which the substrate softens. It is known that texture increases with an increase in deposition temperature.
The prior art textured tin oxide deposition method taught by Kane works well during batch processing. This method, however, is inadequate when applied to continuous processing, that is, when a plurality of substrates are successively moved through the deposition chamber along a belt or roller conveying apparatus. We have found that the method disclosed by Kane tends to produce non-uniform texture in a continuous deposition system and that the texture becomes increasingly non-uniform as the film thickness is increased.
Another disadvantage of the above-described prior art method is that texture is directly related to film thickness. Texture increases as the overall thickness of the tin oxide layer increases. Consequently, an overly thick film with poor optical qualities (i.e., decreased transparency) often results from an effort to increase the texture of a tin oxide layer. Furthermore, the electrical properties of the tin oxide layer vary with the layer's thickness. For example, sheet resistance is inversely proportional to layer thickness. The prior art methods of depositing textured tin oxide layers provide no means for producing, for example, a 15 .OMEGA./.quadrature. layer and a 25 .OMEGA./.quadrature. layer having comparable light-scattering properties. In designing photovoltaic devices, it is desirable to be able to adjust the thickness and texture of the tin oxide film independently of one another. Conventional deposition methods do not provide such a capacity.
The present invention is intended to provide a method of fabricating a substantially uniformly textured layer of tin oxide on a vitreous substrate in a continuous processing operation.
The present invention also is intended to provide a method of fabricating a textured layer of tin oxide on a vitreous substrate in which the degree of texture and thickness of the layer can be controlled substantially independently of one another.
Additional advantages of the present invention will be set forth in part in the description that follows and in part will be obvious from that description or can be learned from practice of the invention. The advantages of the invention can be realized and obtained by the method particularly pointed out in the appended claims.